1. Field of the Invention
The invention relates to the coupling of optical fibers to a light source, through the use of a diamond coupler.
2. Description of the Prior Art
The use of infrared-transmitting optical fibers as internal reflection elements (IREs) for evanescent-wave absorption spectroscopy has been disclosed in the prior art such as, E. Margalit et al, 1987, Proc. SPIE, Vol. 1048, 145-152; Simhony et al, 1988, J. Appl. Physics, Vol. 64, 3732-3724; D. A. C. Compton et al, 1988, Appl. Spectrosc. Vol. 42, 927-979; J. Heo et al, 1991, Applied Optics, Vol. 30, 3944-3951). Fibers made of silver halide, metal fluorides, chalcogenides, and other materials have been successfully used in the prior art for such measurements in combination with broadband illumination from an incandescent or black-body source focused by means of mirrors and/or lenses onto the end of the fiber. This approach has several disadvantages. First, it can be difficult to adjust the position of a lens or mirror properly to align an invisible IR beam on the end of a narrow fiber. This alignment, once achieved, is likely to be very sensitive to external stresses (mechanical, thermal, etc.). Another disadvantage is that substantial reflective loss occurs when light is incident from air, with a refractive index of 1.0, onto the end of the fiber, which typically has an index of refraction between 1.5 and 3.
Antireflection coatings for chalcogenide fibers have recently become available, however they are generally expensive to design and manufacture and somewhat limited in bandwidth.
The most severe limitation of focusing optics operating in air, however, is the relatively small range of optical fiber modes that can be excited. This limits the amount and kind of light energy available for evanescent-wave spectroscopic measurements. Chalcogenide fibers ranging from 50 to 500 micron in diameter, have been used to obtain evanescent-wave absorption spectra of biological samples. The large diameter of typical infrared-transmitting fibers, relative to the light wavelengths they transmit, is calculated to allow the propagation of a large number of transverse optical modes. However, the difference in refractive index between air and the denser optical fiber materials makes it impossible to excite the full range of allowed transverse modes. In particular, this can make it difficult to transmit into the fiber light rays which propagate near the critical angle for total internal reflection.
The full cone of allowed rays cannot be coupled into an optical fiber by means of an optical element which focuses light from a source through air, onto the fiber end. With this type of conventional illumination, the half-angle of the cone of light rays incident on the fiber end is limited by the aperture of the focusing lens or mirror situated between the source and fiber; typical .function./1 optics have a half-angle of only 30.degree.. Even more limiting, however, is the reduction in the spread of this cone of rays upon entry into the fiber, which is a result of refraction at the air-fiber interface. The refraction is quite severe for typical chalcogenide fibers, which have indices of refraction in the range 2.4-2.8. Even with an ideal focusing device with an infinite diameter, the greatest possible numerical aperture (N.A.) would be 1, and the half-angle of the cone of rays propagating within the fiber would be limited to 21.degree.. For .function./1 focusing optics, the N.A. is 0.5 and the cone rays within the fiber has a half-angle of only 10.3.degree..
Even for a focusing device with an infinite diameter, which is physically unrealizable, the maximum incidence angle on the end of the fiber is 90.degree. and the resulting half-angle of the cone of rays propagating within the fiber is 21.degree.. Another way of stating this limitation is: the numerical aperture of any focusing device operating in air or vacuum is less than 1. This limitation holds quite generally for any type of focusing arrangement, as long as light passes through air (n=1.0) for any significant distance between the source and a high-refractive index fiber. In fact, however, infrared optical fibers are themselves calculated to have numerical apertures greater than 1. For example, a commercially-available As-Se-Te fiber has an index of refraction of 2.8 and a numerical aperture calculated to be 2.48. Thus, a focusing optic operating in air is incapable of filling the theoretical aperture of the fiber.
It is obvious from prior art that a focusing optic is entirely superfluous if the source is large enough and/or close enough to the fiber to occupy a full 2.pi. stearadians of solid angle, as viewed from every point on the end of the fiber. It is also obvious that this might be achieved with a source as small as the cross-sectional area of the fiber, if the source were in direct contact with the end of the fiber. However, coupling broadband light into a fiber by direct optical contact with a blackbody source at a temperature above 1000K has not been pursued previously. Among other reasons, this is because optical fiber materials have low melting points. For example, the softening temperature of typical chalcogenide glasses useful for infrared transmission is about 440K, while a broadband infrared source useful for vibrational spectroscopy must generally operate at 1100-1600K. Thus, a thermally-insulating air space has always been required for transmitting light from a source into the fiber without destroying the latter. This air space is required also if a focusing element is present, since the hot source would also damage optical materials that are currently used for infrared lenses or mirrors.
In previously published descriptions of medical laser instruments for transmitting infrared laser energy to a selected part of the body (see A. L. Gentile and D. A. Pinnow, 1979U.S. Pat. No. 4,170,997; M. Seal and W. J. P. van Enckevort, 1988, Proc. SPIE, Vol. 969, 144-152), a diamond window has been used on the output end of an optical fiber, i.e. distal from the light source. In this use, the diamond has served as an inert window to protect living tissues from contact with a metal halide optical fiber. However, the ability of diamond to withstand high temperature gradients was not mentioned, nor was the diamond envisioned as being used to increase the range of optical modes which are transmitted either into or out of the fiber.
As developed for temperature sensing systems (see D. C. Tran et al., 1987, Proc. SPIE, Vol. 843, 148-154; S. O. Heinemann et al, 1991, U.S. Pat. No. 4,988,212), prior art has involved the use of a polished sapphire rod to couple radiation emitted by a hot object to an optical fiber bundle. However, sapphire does not have as extensive a spectral wavelength transmission range as diamond and its index of refraction is lower (1.7 compared 2.8 for a diamond). Furthermore, the optical arrangement envisioned in this prior work would not be capable of transmitting high-order optical modes into the fiber, since air, a low refractive-index medium, intervened between the hot source and the input end of the sapphire rod.
Additionally, economic and transparency considerations limit the size of a useful diamond rod coupler to &lt;2 cm, and it is not obvious from the prior work with large sapphire rods that such a short rod could constitute a suitable coupler. This is because the end of the rod that is in contact with the fiber must be kept near room temperature, while the other end will reach a temperature of 1000-1800K. The resulting extreme thermal gradient might be expected to shatter such a short rod of solid material. Thus, the idea of using a short rod of diamond, and cooling its junction to a fiber or fiber bundle while maintaining a temperature differential of up to .sup..about. 1500.degree. C. across its length, was not obvious from prior art.
Infrared evanescent-wave spectra have previously been obtained using plastic-coated fibers, but only for sample molecules capable of diffusing or dissolving into the coating material until the sample molecules were within several microns of the core fiber material (see V. Ruddy et al., 1990, Appl. Spectros., Vol. 44, 1461-1463). All other previous evanescent wave spectroscopy using chalcogenide optical fibers as IREs has required that the chalcogenide material be directly in contact with the bulk sample. That is, the protective plastic coating had to be removed over at least the measuring portion of the fiber length. This was because only a limited range of transverse modes was present in the fiber, and all of these modes would have been totally reflected at an interface between chalcogenide fiber, having an index of refraction above 2.4, and plastic, with an index of refraction under 1.6. One method for utilizing high-order modes for evanescent-wave spectroscopy has been described previously, involving the use of tapered fibers (see A. Bornstein et al., 1991, Proc. SPIE, Vol. 1591, 256-262; R. D. Driver et al., 1991, Proc. SPIE, Vol. 1591, 168-179). However, this work did not envision making use of these high-order modes for evanescent-wave spectroscopy with fibers that had a thick (&gt;10 micron) lower-refractive-index coating on them. Specifically, it has not been envisioned to use the high-order modes transmitted through tapered fibers for evanescent-wave spectroscopy where the sample contacts only a plastic coating on a chalcogenide fiber.
The instant disclosure provides a direct contact optical coupling for chalcogenide fibers and hot sources through use of a diamond rod as an intermediate medium, thereby overcoming the problems of the prior art.